Development of a sorption cooling test device, using a thermochemical material. M.W.B. Mangnus WET TU/e short internship October 2007

Transcription

1 Development of a sorption cooling test device, using a thermochemical material M.W.B. Mangnus WET TU/e short internship October 2007 Supervisor: dr.ir. C.C.M. Rindt Eindhoven University of Technology Department of Mechanical Engineering Division Thermo Fluids Engineering Section Energy Technology

2 1

3 Development of a sorption cooling test device, using a thermochemical material TU/e short internship October, 2007 Author M.W.B. Mangnus Supervision Dr.ir. C.C.M. Rindt Eindhoven University of Technology Department of Mechanical Engineering Division Themo Fluids Engineering Energy Technology group

4 3

5 Abstract Due to the increasing demand of energy and the changing climate (greenhouse effect), the need for sustainable solutions plays an important role in developments. Sorption cooling systems are environment friendly compared with traditional systems. They employ safe and are mainly non-polluting. Another advantage of sorption systems is that they can be driven by low-grade energy such as waste heat or solar energy. The goal of this project is to develop a cooling test device using an environment friendly thermochemical material (TCM) as adsorbent. A literature research has pointed out several candidate TCM s, including zeolites. Zeolites have several advantages, among others that they are safe in use and that the adsorbed water can be expelled from the lattice without destruction of the structure. The best fitting material, a synthetic zeolite 13X is chosen as adsorbent. It has proven itself in other existing adsorption cooling systems. Some successful utilizations are: a sun driven refrigerator for developing countries and a heat/cooling system in Germany. A sorption cooling systems has been build as test device. Like the most of these kinds of systems, it makes use of the evaporation of water. To reach a high evaporation/cooling rate the device operates at the vapour pressure of water, and the air is being forced to circulate through an adsorber and evaporator. With the test device, the cooling principle has been proven. A temperature decrease of four degrees Celsius can be reached. The performances of the system can be optimized with relatively simple adaptation. 4

6 5

7 Contents Abstract... 4 Contents Introduction Thermochemical materials Background information of zeolites Zeolites as adsorbing material Existing zeolite sorption cooling systems Solar adsorption refrigeration using zeolite and water Open sorption system in Munich The cooling system The cooling cycle Atmospheric pressure system Low pressure system Experimental results of the test device Conclusions and recommendations Bibliography List of figures APPENDIXES Appendix A Mollier diagram Appendix B Short manual of the test device

8 7

9 1. Introduction There are increasingly more heat producing equipments in buildings and people have a higher comfort demand than in the past. As a consequence, the cooling demand of buildings is getting bigger. Due to this increasing demand of energy and the changing climate (greenhouse effect), the need for sustainable solutions plays an important role in developments. Nowadays, air-conditioning systems mostly use conventional energy sources and operate according to the Carnot principle. Investigation has to be done on alternative cooling methods, which are more sustainable. Evaporative air conditioning is the cooling of air by the evaporation of water. When water evaporates into the air to be cooled, simultaneously humidifying it, it is called direct evaporative cooling, the oldest and most common form. An example of a well known conditioned system is the human body. When the body becomes to hot, it will release fluid (sweat). The sweat evaporates by extracting energy from the body (heat of evaporation Δh v ). Another example of the same principle was known to the ancient Egyptians, where porous earthen vessels were used for keeping water cool. In many country s, the moisture content in the surrounding air is too high and an open cooling system operating on this way is not profitable enough. When the air to be cooled is kept separate from the evaporation process, and therefore is not humidified as it is cooled, that is called indirect evaporative cooling. This study concerns one specific indirect evaporation cooling system, namely the sorption cooling systems. The water content of the air in the system will be largely adsorbed by the adsorber. This will cause the water vapour flow from the evaporator to the adsorber [Figure 1.1]. The adsorption material increases in temperature. Figure Principle of a sorption cooling system Sorption cooling systems are environment friendly compared with traditional systems. They are safe in use and the refrigerants are non-polluting. Another advantage of sorption systems is that they can be driven by low-grade energy such as waste heat or solar energy. A Thermo Chemical Material (TCM) is usable as adsorption material. By heating a TCM, water vapour releases and the TCM is in dehydrated condition. In this project the dehydrated condition of a TCM will be used to cool down an amount of air. The goal of the project is to build a cooling test device, using a TCM. The device can: be used for educational prospects increase knowledge about TCM sorption in a manageable environment be optimized and scaled up to a working cooling system (possibly a conditioning system). contribute to a more sustainable world 8

10 The desorption is not part of the project. It can be realised by pre-heating, which can finally be done in practice by solar energy or residual heat. In chapter 2 general background information is given about TCM s and in special the zeolites. Zeolites are minerals with a micro-porous structure. In chapter 3 several existing adsorption cooling systems are discussed and compared which each other. In chapter 4 the construction and experimental results of the cooling test device are discussed. In chapter 5 conclusions and recommendations are presented. 9

11 2. Thermochemical materials Thermochemical materials (TCM s) can undergo reversible chemical reactions. The reaction is energy demanding in one direction and energy yielding in reverse direction. The properties of these materials can be used for the storage of heat. The basic principle of a TCM is: Hydrated TCM + Heat Fluid/Gas + Dehydrated TCM Heat will be released when water is added to the dehydrated TCM [Figure 2.1]. This is the hydration step, which is exothermic. Some of the water evaporates due to the heat production; this is visible in the picture. The dehydration step is endothermic, because heat is necessary to evaporate the water from the TCM. There are several suitable materials on the market, each with there own advantages. For seasonal storage an investigation has been done by Visscher [1]. The outcome of this investigation is partly used by van de Voort [2] to characterize the best fitting material. General background information is given and detailed analyses are determined. Figure 2.1 Adsorption of water in zeolite One class of thermochemical materials are the crystalline salt-hydrates. An inert molecule (like water) can be incorporated into the crystal lattice of the hydrate. These hydrates are separated in two groups: the gas hydrates and the zeolites. Zeolites have the advantage that the adsorbed water can be expelled from the lattice without destruction of the structure. Zeolites are chosen for this study due to this property and their safe employ. 2.1 Background information of zeolites Zeolites are minerals with a uniform micro-porous structure and largely with identical properties. The most important properties are that they can act as: absorbers, ion exchangers, molecular sieves and catalysts. For each different situation the most suitable zeolite has to be found or developed. There are natural and synthetic zeolites on the market. For this study, the adsorption property is important. Zeolites can store energy in the form of heat due to the sorption effect, as shortly described above. Zeolites are alumino-silicates, a subgroup of silicates. Silicates are the most occurring minerals in nature and consist out of the most occurring elements: silicon and oxygen. Zeolites have a crystalline structure with very regular, three dimensional openings, linked with each other. In these opening the mineral can adopt several inert molecules (like water), which can freely move in the lattice [3]. 10

12 Chemical composition The basic element of zeolite consists out of a tetrahedron of four oxygen elements and a central silica or aluminium element. Each oxygen part connects two tetrahedrons with each other. Besides, the mineral is often built from several other elements. The general chemical composition of zeolite is: Mg /no Al O ysio wh O With: 2 y 10 n Value positive charge atom M w Percentage water in pores Structure The atoms in zeolite are perfectly well regulated. There are openings present in the structure, they can reach 50% of the total volume. Dependent on the chemical composition, these spaces can adopt several ions and water molecules [Figure 2.2] These ions or water molecules are not bounded with the structure. An ion will be exchanged with another ion when it fits better in the structure (preference selection). Stability Stability or cyclability is a very important aspect when talking about efficiency and reliability. The structure of zeolites can be destroyed in the desorption step. Exposure to high temperatures can degrade the capacity of some zeolites (above 70 C). Applications Adsorption Zeolites can adsorb several molecules (like: water, nitrogen, ammonia, lead, chromium), without reacting with them. There is no change in structure. For water a graphical representation is given in figure 2.2. The absorption application is used for example in the case of air purification, water filtering and drying of gasses. The use of zeolites for this kind of applications can contribute to energy savings, an increase of production capacity and lower releases of unpleasant smells. Ion exchange It is possible to remove unwanted ions from a solution with loaded zeolites. If the zeolites are loaded with ions which don t fit as well in the structure as the unwanted ions in the solution, exchange with them will take place. The ion exchange property is used for example in water softeners, washing-powder, but also for waste water purification and radioactive water. 11

13 Molecular sieve Zeolites do have porous structures which make them applicable as sieves [Figure 2.3]. Molecules with a bigger size than the open spaces in the structure will not be absorbed. In the process industry these zeolites are widely used. An example is the increasing of the octane content in gasoline. Catalyst Due to the porous structure of the zeolites they have a very big surface, where reactions can take place. They can act very well as catalysts and are very often used in the petrochemical industry for the refining of oil. Types In nature, there are 48 different types of zeolites known and there are over 150 synthesized zeolites. Natural zeolites can be found on several places on earth, they can be obtained by conventional mine extraction technologies. Synthetic zeolites are produced to match the structure with their purposes as good as possible. An optimization can save huge amounts of money, especially in the petrochemical industries. 2.2 Zeolites as adsorbing material As described in the paragraph above, zeolites can be used as a thermochemical material. The advantage of zeolites in comparison with other TCM s is, that it is a relatively save material, environment-friendly and it has a uniform pore diameter [Figure 2.4]. They can adsorb molecules with a smaller diameter than their pore diameter. The pore diameter of water is 0,265 Å (1 Angstroms = 0,1 nm). There are several applicable zeolites on the market, each with there own advantages. Figure 2.4 Pore diameters absorbing materials The selected mineral for this study has to fulfil the following demands: High absorption percentage of water Good hygroscopic properties (ability of a substance to attract water molecules from the surrounding environment) Low temperature desorption possible (residual or solar energy has to be usable) Low degradation rate of the material (good cyclability) A synthetic zeolite has proven itself for these kinds of aspects, namely type 13x (NaX). This molecular sieve is a highly porous alkali alumino-silicate. The pore openings in the crystals have a diameter of about 10 Å (or 1 nm). 12

14 It has a high capacity for water and carbon dioxide and is widely used for air purification in cryogenic air separation plants. It is also used for the removal of Hydrogen Sulphide and mercaptans (a group of sulfur-containing organic chemical substances) from natural gas and LPG. Some technical parameters of zeolite 13x are given in table 2.1 Typical Properties Water adsorption 10% RH 25ºC 23,0 % w/w Water adsorption 80% RH 25ºC 29,0 % w/w Carbon Dioxide adsorption 265 Pa, 25ºC 5,0 % w/w Carbon Dioxide adsorption 3330 Pa, 25ºC 19,0 % w/w Total volatiles 950ºC 2,0 % w/w Residual water content 575ºC 1,5 % w/w Crush Strength - 40 N Bead Size - 1,6-2,5 mm Bulk Density g/l Physical Properties Melting Point - > 1000 ºC Boiling Point - > 2000 ºC Flash Point - Not applicable Table 2.1 Parameters zeolite 13x Zeolite 13x can absorb roughly 25 % w/w of water. This is an important fact for the design of the system (proportion of water to zeolite). Melting takes place above 1000 C, so desorption can be done at high temperatures. 13

15 3 Existing zeolite sorption cooling systems In this chapter, several existing sorption systems will be handled which make use of zeolite as adsorber. 3.1 Solar adsorption refrigeration using zeolite and water Refrigeration of food is of great importance to prevent spoiling. Especially in developing countries where electricity is not available, solar cooling systems can be an outcome. Radiation is often one of the most easily accessible energy sources in those areas. An adsorption system has been made for those circumstances. The driven energy is radiation and the mediums used for cooling are zeolite and water. The system works as described in chapter 1. In figure 3.1 another overview of the working process including the desorption is schematically reflected. The cooling cyle is driven by hygroscopic forces (blue arrows). The humid air will be adsorbed by the zeolite. The water starts to evaporate and the temperature decreases in the evaporator. The temperature of the adsorber increases. The reverse cycle is driven by solar energy (red arrows). The necessary heat for desorption will be delivered by radiation from the sun. The zeolite is located in a vacuum tube solar collector. By enough sunlight it can be heated up to 180 C. The system employs at a low pressure to increase the evaporation speed of the water. When the vapour pressure equals the saturation vapour pressure of water, the water starts to boil. This is at atmospherically circumstances (1 bar) at about 100 C. By lowering the pressure, the boiling point will decrease. The result presented in figure 3.2 shows that a water temperature of 0 C can be reached. At first desorption takes place by radiation. The temperature of the Figure 3.1 Solar adsorption refrigeration Figure Typical result of experimental cycle 14

16 zeolite reaches 170 C. After desorption, the zeolite cools down by the surrounding temperature. The adsorption phase starts; due to the adsorption the temperature of the zeolite increases and due to the evaporation the temperature of the water decreases. Qcool There is a prototype made of the test setup with: COPcool = = Qsolar Q solar is the total amount of radiation energy and Q cool is the total amount of energy delivered for evaporation [Figure 3.1]. For more information one is referred to Kreussler [5]. 3.2 Open sorption system in Munich In Germany a system has been installed to heat a school in winter and cool a jazz club in summer. The adsorbing material is zeolite 13x. The difference with the systems treated before is that this system is an open sorption system. An open system adsorbs the ambient air, instead of a closed system that is totally separated from the air to be cooled. Air heating system The heating system is used to lower the power peak demands during day time. The system is loaded at times when there is a low energy consume [Figure 3.3]. The water content of the zeolite will be lowered by using indirect steam. The waste heat of charging process and the heat of condensation is used to warm the school at night. At day time, the air inside the building will be transported through the zeolite. Before entering the zeolite buffer, the air passes a humidifier. The humidified air leaves the mayor of the humid in the zeolite buffer, due to hydroscopic attraction. The thermal energy is transferred to the heating system of the school by a heat exchanger. Qads + Qcond The COPheat = = 0.92 Qdes This coefficient is defined as the ratio of thermal energy supplied to the building (heat of condensation Q cond and adsorption energy Q ads ) and thermal input for charging the storage (desorption energy Q des ). Figure Charging of the heating system Figure 3.4 Discharging of the heating system 15

17 Air conditioning system The conditioning system is an open sorption system. Outside air flows through a dehydrated zeolite bed. Most of the water content of the air will be adsorbed by the bed. The dried air has an increased temperature. This temperature will be lowered in a heat exchanger, where its thermal energy will be exchanged with the lower exhaust air from inside the building. Finally the air will be humidified to a comfortable value. Owed to the humidifying, the temperature of the air is lowered too. This principle will be explained in chapter 4. The exhausted air will also be humidified, before it enters the heat exchanger. This is done to reach the coldest temperature as possible (at the dew point). Schematically the system is reflected in figure 3.5. Figure 3.5 Cooling system Qcool The maximum COPcool = = 0.87, reached by a low desorption temperature of 80 C. Qdes With Qcool = ΔH cool m dt, the value ΔH cool is the enthalpy difference of the air stream caused by the adsorption within the zeolite tank and the cooling effect caused by the cold recovery [Figure 3.5] *. Q des is the same as in the heating system, namely the thermal input for charging the system. For more information one is referred to Hauer [8]. * Remark: This COP value is not direct comparable with the solar system 16

18 17

19 4 The cooling system 4.1 The cooling cycle For adsorbing cooling systems a Clapeyron diagram (ln P vs 1/T) is useful to explain what thermodynamically happens in a cooling cycle, see figure 4.1. The cycle consists of four processes: pre-heating, desorption, pre-cooling and adsorption. The ratio between the mass of adsorbed water and the mass of dry zeolite is called the refrigerant uptake (x). 1 2: In state one, the zeolite is cold and saturated with water (x=x max ). Heat is added until the minimum desorption temperature is reached at state two. The pressure and temperature are increased. Figure 2.2- Adsorption of a water molecule in zeolite Figure The micro-porous molecular structure of a zeolite 2 3: At state two, the pressure of the system becomes equal to the saturation pressure corresponding to T con. Figure 4.1 Ideal adsorption cooling cycle (Clapeyron diagram) Desorption starts and the water is condensed (isobar), normally in a condenser. This step proceeds until the adsorbent temperature reaches the maximum available desorption temperature and the water uptake reaches the cycle minimum (x min ). 3 4 The system has to cool down by the ambient, before the adsorption process should start. By cooling down the system, the pressure also decreases. 4 1 After all, the adsorption and thus the cooling step can take place. The condenser will act now as evaporator. Due to the evaporation energy will be subtracted from the system. The cooling process stops, when all the water is evaporated. The cycle described above, can be divided in two parts, see figure 4.2. Part one is Figure 4.2 The two parts of the cooling cycle 18

20 energy demanding, the pre-heating and desorption group. Part two is the pre-cooling and adsorption group. Remind that only the second group (figure 4.2b) is part of the project. 4.1 Atmospheric pressure system The zeolite system from paragraph 3.2 employs under atmospheric pressure, as it is an open system. A big advantage of such a system is that small leakages are not a big part and the design can kept simple and cheap. Although the test device has to be a closed system (because open system are not easily usable for simple tests), it can still be competitive. The principle behind the cooling process can best be explained with Mollier diagrams. These diagrams are used to design compressors, power plants, steam turbines, refrigeration systems, and air conditioning equipment. They visualize the working cycles of thermodynamic systems, often they are called h-x diagrams. The enthalpy of moist air [J/kg] versus the water vapour content [g/kg] of dry air is plotted for several air conditions (temperature, relative moist content). A simplified version of a mollier diagram is presented in figure 4.3, for a more detailed diagram is referred to Appendix A [9]. Figure 4.3 Example Mollier diagram R H = 9 0 % R H = 1 0 % E n t h a l p y [ k J / k g ] W a t e r c o n t e n t [g /k g ] Table 4.1 Air at 30 C To calculate the feasibility of a system operating under atmospheric pressures, simple calculations can be satisfying. The water content and the enthalpy of air at 30 C at a relative humidity of 90 and 10% is gathered from the Mollier diagram, see table

21 Concept A possible system is presented in figure 4.4. There is a circulation pump installed to drive the evaporator and to transport the water vapour faster to the adsorber than with natural convection. The amount of air that is circulated will be a laminar flow of 1,2 m 3 /h. The pressure drop over the zeolite and in the evaporator will stay low. The smaller the pump, the lower the energy consumption and thereby unwanted heat production. The pressure drop over the zeolite is comparable with activated carbon, see figure 4.5. The particle sizes, flows, and temperatures are almost similar. The pressure drop is in the order of mbar and negligible. The evaporator itself is nothing more than a buffer with water where air is blown through a membrane at the bottom. Due to small openings in the membrane the dry air can pass, but the water can not. The membrane is called so on a fine bubble membrane diffuser, because of the small air bubbles which are produced when the air is blown through it. These bubbles cause a bigger evaporation area. Evaporation is a surface phenomenon, water molecules can escape when their kinetic energy is high enough to overcome the intermolecular forces (van der Waals). The remaining molecules have a lower kinetic energy average and the temperature decreases. The pressure drop over the membrane and water column is also in the order of mbar and will be neglected too. Figure 4.4 Possible atmospheric pressure system Figure 4.5 Pressure drop zeolite bed Feasibility Assumed is that the process is totally adiabatic, the air is an ideal gas (Pv=RT) and the system is perfectly well insulated. The system uses an air flow of 1.2 m 3 /h with a density of 1.2 kg/m 3 to evaporate 0.2 kg water with a temperature of 30 C and a specific heat of 4.2 kj/kg.k. An approximation of the time to cool the water with 5 C is made below: 20

22 Δ h = T2 T1 c dt p T2 mδ h = M c dt = Mc ΔT T1 p p ( ) ( )( ) McpΔ T = m hout hin = m x x h h McpΔT 0, 2 4, 2 5, 0 m = = = 3,5 kg x x h h 3 3 ( 2 1)( 2 1) ( 2, , 4 10 )( 92, 4 36, 8) (4.4) After 3.5 kg of air has circulated over the evaporator, the water has cooled 5 C. t = m = m ρ = 3,5 2,5 hour m m 1, 2 1, 2 äir air (4.5) Under atmospheric pressures, it will take almost 2,5 hours to cool 0,2 kg of water with 5 C. Under these circumstances the setup would not cool at all, because of all the unwanted energy entering the evaporator in practice (Q pump and Q surrounding ). When scaling up this system, it might be profitable as cooling system. All the heat producing equipment has to be kept outside (adsorber and pump) the area to cool. Further investigation has to be done to examine this possibility. 4.2 Low pressure system The system above is not suitable as a test device. If the evaporation speed increases, it possibly will. To realize this, one can increase the kinetic energy of the molecules. Unfortunately this will lead just to a higher temperature. When the vapour pressure reaches the ambient pressure, the liquid starts to boil. This creates the possibility to boil water at low temperatures by lowering the surrounding pressure. The saturation pressures for several temperatures are showed in the Mollier diagram (appendix A). A simple diagram is presented in figure 4.6. The solar refrigerator from paragraph 3.1 employs on low pressure. When building a vacuum evaporator, the energy necessary to keep the water boiling will be delivered by the material and the water itself. The temperature of the total evaporator will decrease. Figure 4.6 Vapour pressure of water 21

23 Concept Almost the same concept as in paragraph 4.1 will be handled. The membrane is excluded, because the evaporator is driven now by the boiling process. The concept of lower pressure has been proven to work in paragraph 3.1 and will be used as test device. With the test device the sorption cooling principle has to be demonstrated. It has to be a clear design, so it can be used for education. No exotic materials or complex components will be part of it, refitting and optimizations can easily be carried out. The test device is shown in figure 4.7. Figure 4.7 Test device Construction The device consists of a zeolite buffer (adsorber), a water buffer (evaporator), a circulation pump (for low pressures), a vacuum pump, a water tank and a cooling spiral. The zeolite can be dehydrated with the blow drier. The connections between the equipment are made with standard 15 mm cupper pipes. These are chosen for economical reasons, the metal conduct unfortunately heat. The red lines in the picture are the hot air pipes, the blue ones are the cold pipes. A temperature sensor is placed in the adsorber and one in the evaporator. A pressure sensor is placed to monitor the vacuum. Amounts To cool, there has to be added 1 kg of zeolite in the adsorber and 0.3 kg of water at room temperature in the evaporator. Roughly 25 % w/w water adsorption can be reached by the zeolite, see paragraph 2.2. The evaporator and adsorber are build from stainless steel. The evaporator has a total surface (outside) of 0.14 m 2 and a volume of 3.81 x 10-3 m 3. The density of stainless steel 8 x 10 3 kg/ m 3. For operating instructions one is referred to appendix B. 22

24 Working principle With the vacuum pump, the required pressure can be reached [figure 4.6]. The water vapour will be transported through the zeolite bed (the adsorber). The zeolite is highly hygroscopic and adsorbs the water. The temperature of the adsorber and the water will rise due to the adsorption energy Δh adsorp. The adsorption energy is the sum of the heat of evaporation Δh evap and the binding energy Δh bind (Δh adsorp = Δh evap + Δh bind ). For this reason the adsorber is placed in a water tank. The cooling spiral cools the dried air flow. This stream is pumped with a special diaphragm membrane pump for low pressures into the evaporator. The dry air is moistened in the evaporator, which cools down (Δh evap ). As air cooling system, it has to be scaled up and the evaporator has to be placed inside the area which has to be cooled. The other heat producing equipments have to be placed outside that area. 4.3 Experimental results of the test device The low pressure system concept is the most suitable as explained in the previous paragraph. The experimental results are done to prove the cooling principle. Further investigation and optimization of the device is out of scope. The goal of this first set of experimentations is to obtain information about the performances of the system. The experiment has been done twice; the reached temperature difference was the same. Figure 5.1 Measured temperature decrease of the evaporator A slight decline of temperature has been reached, as can be seen in figure 5.1. The first hour, the temperature decreases 3 C. After that it takes two hours to cool 1 C, stagnation takes place. The evaporator is in equilibrium, the incoming energy is equal to the outgoing energy. 23

25 5 Conclusions and recommendations Conclusions The goal of this project was to develop a test cooling device using an environment friendly thermochemical material (TCM). A literature research has pointed out several candidate materials, including zeolites. The best fitting material, a synthetic zeolite 13X is chosen as adsorbent. For small scale applications like the test device, an atmospheric operating pressure seems not to be the best choice. Evaporation of water will cause cooling, but it takes to much time. A better result can be reached, when lowering the pressure. Boiling at room temperature has to be the starting point for the experiments. To further increase the evaporation speed, a circulation of air over the adsorbent and evaporator is desirable. This circulation will be realized by a diaphragm membrane pump; these are applicable at low pressures. The heat producing adsorber has to be cooled, even as the heated dried air. The adsorber and a cooling spiral (for the air), are placed in a water tank. The final test device cools down the evaporator with four degrees Celsius. Recommendations The device can be optimized with three adaptations: 1. Replacement of the heat producing diaphragm membrane pump 2. Reduction of conduction by using pipes, made from low conducting materials 3. Reduction of convection from the ambient to the system by using better insulation 1. Pump replacement The pump is now installed behind the cooling spiral, see figure 6.1. This is done because the membrane is very sensitive and the occurring temperatures were unknown. The pump produces unwanted heat, this results in a lower efficiency. To overcome this problem party, the conduction has been reduced by using a polymer hose between the pump and the evaporator and an extra fan is used to cool the pump somewhat. Anyhow there is still too much convection from the pump to the circulating air. When replacing the pump before the spiral, the heat production of the pump can be exchanged with the water. Figure Replacement of the diaphragm membrane pump 24

26 2. Using lower conducting pipes All the pipes in the device are made from copper; these are chosen for economical reasons. Unfortunately metal conducts heat very well. Replacement of all the pipes which are insulated at the moment (the cold pipes, see figure 4.7), by low conducting materials like polymers will result in a higher efficiency. 3. Using better insulation The device is somewhat insulated to restrict the convection from the ambient to the system. The insulation can be done more accurate and there are better types on the market. Other recommendations: Redesigning the evaporator will be necessary when a lower heat capacity is wanted. With a lower heat capacity of the evaporator, lower temperatures can be reached. To realize this, one can for example: use another material than stainless steel or lower the wall size. Remark that the total energy subtraction can still be the same, because this is dependent on the amount of evaporated water. If the device will be used for further investigation, it is advisable to use more (accurate) sensors. An infrared thermometer can be useful to measure the other available parameter (temperature of: water tank, pipes etc.) There also have to be paid attention on the desorption. The blow drier is applicable, but probably not the best user-friendly solution. 25

27 Bibliography [1] VISSCHER, K., VELDHUIS, J.B.J., OONK, H.A.J., VAN EKEREN, P.J., BLOK, J.G., Compacte chemische seizoensopslag van zonnewarmte, ECN-C ,2004 (2004) [2] VAN DE VOORT, I.M., Characterization of a thermochemical storage material, WET (2007) [3] LOOMAN, R.I.J., Deel 1: toepassingsmogelijkheden van zeoliet in de bouw, (2004) [4] LOOMAN, R.I.J., Deel 2: de invloed van het hygroscopisch vermogen van zeoliet op de vochtbalans, (2004) [5] KREUSSLER, S., BOLZ, D, Experiments on solar adsorption refrigeration using zeolite and water, Laboratory for solar Energy, University of Applied Sciences Luebeck. [6] DAWOUD, B., A hybrid solar-assisted adsorption cooling unit for vaccine storage, Chair of Technical Thermodynamics, Aachen (2006) [7] LIU, Y., LEONG, K.C., The effect of operating conditions on the performance of zeolite/water adsorption cooling systems, School of Mechanical and Production Engineering (2004) [8] HAUER, A., Thermal energy storage with zeolite for heating and cooling applications, Center for Applied Energy Research (2002) [9] KULING, H.TH.C., Luchtbehandelingstehniek, Intechnium (2001) [10] IVRINE, T.F., HARTNETT, J.P., Steam and air tables in SI units: including data for other substances and a separate Mollier chart for steam, London: Hemisphere (1976) [11] WATT, J.R., KORAL, R.L., CROW, L.W., GREENBERG, A., Evaporative air conditioning handbook, London: Chapman & Hall (1986) [12] SAAD, M.A., Thermodynamics : principles and practice, Upper Saddle River: Prentice Hall (1997) 26

28 27

29 List of figures Figure 1.1 Principle of a sorption cooling system Figure 2.1 Adsorption of water in zeolite Figure 2.2 Figure 2.3 Figure 2.4 Figure 3.1 Figure 3.2 Figure 3.3 Figure 3.4 Figure 4.1 Figure 4.2 Figure 4.3 Figure 4.4 Adsorption of a water molecule in zeolite [Source: Bibliography 3] The micro-porous molecular structure of a zeolite [Source: Pore diameters absorbing materials [Source: Bibliography 3] Solar adsorption refrigeration [Source: Bibliography 5] Typical result of experimental cycle [Source: Bibliography 5] Charging of the heating system [Source: Bibliography 8] Discharging of the heating system [Source: Bibliography 8] Ideal adsorption cooling cycle (Clapeyron diagram) [Source: Bibliography 6] The two parts of the cooling cycle [Source: Bibliography 6] Example mollier diagram [Source: Possible atmospheric pressure system Figure 4.5 Figure 4.6 Figure 4.7 Pressure drop over zeolite bed [Source: Vapour pressure of water [Source: /esspm/clim_wat/client_edit/climate_water/slides/vapor_p.gif] The test device Figure 5.1 Measured temperature decrease of the evaporator Figure 6.1 Replacement of the diaphragm membrane pump 28

30 29

31 Appendixes Appendix A Appendix B Mollier diagram... Error! Bookmark not defined. Short manual of the test device... Error! Bookmark not defined. 30

32 31

33 Appendix A Mollier diagram This diagram is used to design several thermodynamic systems. 32

34 Appendix B Short manual of the test device The test device is schematically shown below: Cooling At first, all the valves have to be closed. Switch the vacuum pump on till the pressure sensor indicates the right pressure (dependent on the ambient temperature, see figure 4.6). An extra water buffer from glass has been installed between the vacuum pump and the pressure sensor. When the water in this transparent buffer starts to boil, the right pressure has been reached. Attention: Open the valves in following order to prevent blockage of zeolite in the vacuum and circulation pump Open the valve number 1 and 3 and wait till the vacuum is recovered Open slowly valve 7, if it will be opened to fast zeolite will suck into the system. Wait till the vacuum is recovered. Open valve 5 and 6 and turn the circulation pump on The evaporator starts to cool. 33

35 Desorption The blow drier can be used to dehydrate the zeolite. All the valves have to be closed, except for valve 8. Before connecting the drier, a cap has to be unscrewed. Never put the drier at full power (half should probably be enough). Keep paying attention on the blow drier during desorption. Local temperatures of 500 C can occur! The desorption time still has to be investigated. Teflon tape has been used to hermetically seal the system. The melting point of teflon is 327 C, keep this in mind and stay below this point. Renewing the zeolite To renew the zeolite all the valves have to be closed, except for valve 8 and 9. There are two possibilities: using compressed air unscrewing of the temperature sensor and using a small stick Tip: When using compressed air, use a net to collect the zeolite (diameter > 1.5 mm). Refitting or resealing After resealing or refitting the system, use an overpressure to detect possible leakages. There is a special spray on the market for this (In Dutch: gaslekzoeker). When spraying on a leakage, a kind of soap bubbles will arise. When using the compressed air, use a reducible air valve if possible. The standard occurring pressure in the air system is 8 bar (absolute). Start with a lower pressure and slowly increase when no leakages are detected. Attention: Remove the pressure sensor before applying an overpressure. The sensor is a vacuum sensor!! After all: This system is developed in a relatively small time period. Some assumptions have been made, common-or-garden material has been used and the vacuum pump contains freely rotating parts. To safety use this device, keep focussing! 34